Washington state has approved gay marriage, joining Maine and Maryland as the first US states to pass same-sex marriage by popular vote.

Voter returns released since election night show Referendum 74 has maintained its lead. Opponents conceded the race Thursday, while supporters declared victory a day earlier.

Zach Silk, a spokesman for Washington United for Marriage, called it a "historic day."

"We have always understood that there are good people on the other side of this issue," he said in a statement issued Thursday. "Yet, we remain confident that once people see how much marriage matters to families, they will realize that the love and commitment that marriage embodies only strengthens families, neighborhoods and communities."